It seems many people get confused about the differences (and similarities) between "colloquial" and "slang", so what exactly does each term apply to?

But to be even more thorough it seems to me we can also include a couple more terms which are often applied to language and arrange them into a scale from most acceptable to least acceptable: "informal" > "colloquial" > "slang" > "vulgar"

Are any two synonyms? Do they all overlap semantically or only with their closest neighbours? Or should some of them be seen as independently variable aspects of a word or utterance?

Finally, could each term apply equally as well to a word, a phrase, and a grammatical or syntactical usage?

EDIT

Perhaps I should've also included "nonstandard" - not sure whether it belongs to the left or the right of "slang" though.

4 Answers
4

[Edited, with examples:] There is some overlap between these terms. People will often even disagree whether a certain expression is best considered informal, or rather colloquial, etc. This is merely an attempt at cataloguing possible associations. If you have suggestions for improvement or refinement, do not hesitate.

Informal:

This is the broadest, most neutral word. It just means that speech or writing is on the lower side of the formal–middle–informal spectrum. In informal situations, when your conduct is relaxed in all respects and etiquette matters less, you will use informal language accordingly.

Apart from that, it is neither negative nor positive; that's why it is the best term if you don't want to sound disapproving (and if colloquial is not an option).

There are various degrees of (in)formality: it is usually not a yes–no distinction.

Colloquial:

Oxford English Dictionary: 2.spec. Of words, phrases, etc.: Belonging to common speech; characteristic of or proper to ordinary conversation, as distinguished from formal or elevated language. (The usual sense.)

This is quite close to informal.

It is mostly used with speech rather than writing, though not necessarily so.

The word is also slightly stronger on average than informal (i.e. more informal).

It suggests a yes–no qualification: saying more colloquial is not so common.

It is usually neither positive nor negative, nor felt to be lower class.

However, the euphemism "colloquial at best" is often used to mean that it is bad style, referring to a colloquialism used in the wrong setting.

Slang:

Slang can be a noun or an adjective; slangy means "resembling or constituting slang".

It is more often negative than positive—but it can still easily be positive.

In the formal–middle–informal spectrum, it is more informal than colloquial or informal.

The word slang itself is a bit informal, while the other words on this page are not.

Oxford English Dictionary: 1. a. The special vocabulary used by any set of persons of a low or disreputable character; language of a low and vulgar type. [notice vulgar used ambiguously]

Originally, slang was language associated with low socio-economic class or character, and it is still used with that connotation, though by no means always.

A secondary sense has developed, that of general "group talk" in a mildly disapproving or mocking way—even if this group isn't lower class. This sense is now arguably more common than the first. It is often used ironically, as in lawyer slang.

A tertiary, entirely neutral sense, "any kind of non-standard group talk", is now commonly used in academia.

Vulgar:

This means literally "of the people".The Oxford English Dictionary describes its development through the ages:

I. 3. Commonly or customarily used by the people of a country; ordinary, vernacular. In common use c 1525–1650; now arch.

II. 9. Belonging to the ordinary or common class in the community;
not distinguished or marked off from this in any way; plebeian

II. 13. Having a common and offensively mean character; coarsely commonplace; lacking in refinement or good taste; uncultured, ill-bred.

It can now be used to describe language in two ways:

The old-fashioned sense is as (II. 9.) above. It is still in use in dictionaries, but less frequent elsewhere.

The modern sense is close to (II. 13.), "obscene" or "filthy" to a greater or lesser degree; the lower classes were supposed to be liable to such language, and this sub-sense of (1.) came to dominate the word. So this is obviously even less formal than slang in its lower-class sense. In dictionaries, vulgar could be (1.) or (2.).

I will give a few examples, best description first:

That ain't right.

slang

informal

vulgar (1. of the common people), old-fashioned label

perhaps colloquial

I will try and convince her.

slightly informal

colloquial

some might call this slang or vulgar (1.), but it isn't felt to be connected with lower class by most people, nor with certain specific groups

That sucks.

slang

vulgar (2. obscene), old-fashioned label, because suck has lost its sexual connotation for many people

vulgar (1. of the common people)

informal (a bit too general)

colloquial is possible, but not the best choice

If I was rich, I'd go to London.

informal

colloquial, but many people "would never say was", in neither speech nor writing

@Cerberus: I was actually thinking of "vulgar" to mean "rough", "coarse", "unrefined" and that the "obscene" or "offensive" sense was incorrect, but it seems I am wrong and the term is quite fuzzy despite its use in dictionaries. I also thought of "slang" as a specific kind of trendy invented terminology not approved of by others and hence different to "coarse", "rough", "vulgar" though often one feature of that kind of unrefined speech.
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hippietrailJun 14 '11 at 1:11

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I believe "colloquial" has connotations of "provincial" or "regional", i.e. informal language which is peculiar to a certain geographical area.
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Ben VoigtJun 14 '11 at 1:18

@hippietrail: Yeah these terms are quite complicated and fuzzy. Different people will describe them differently. I think you're right that the group thing is sometimes (not always) an added connotation to slang.
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CerberusJun 14 '11 at 1:23

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I don't want to discredit the thoroughness of this answer, but some of this is quite wrong; the definitions are either evasive, inaccurate, or far too generalized. I don't feel this touches on the shades of meaning the question was looking for.
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HaLJun 14 '11 at 1:36

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@Cerberus: "slang' is definitely not lower-socioeconomic specific. Read any P.G.Wodehouse to get a headful of upper-crust slang. 'slang' is closely related to 'jargon' and 'argot', specialized vocabulary for a subset of people within a dialect. Also, "ain't" is not slang (it's not specific to a subset of the dialect it occurs in). It is informal, but it is not old-fashioned.
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MitchJun 14 '11 at 17:31

Informal - This is directly related to register. It is mostly dictated by social status. One would use formal language in formal setting, such as business functions or any time when you are speaking with members of high/higher society.

Colloquial - This is geographical. There is only one language in England: English. However, two people from different cities might have quite notable difficulty understanding each other in conversation due to colloquialisms. They are often related to the history of the given place and can be influenced by things such as prevalent industry, local surroundings and historical events.

Slang - This tends to be more social. The understanding of slang is usually restricted to a group of peers. This could be a small group or a large group. They could be from very different places and backgrounds. Slang is formed more through mutual understanding and often to intentionally create an element of exclusivity. Because of all these factors, slang tends to change constantly and often does not last long enough to enter into common usage. Though it's a slightly lazy example, consider how teenagers speak. Each generation tends to have it's own slang. It is not constant. It exists for that group of people at that time. It is essentially a type of jargon.

Vulgar - This is a little different to the other terms. Each of the others refers to a style of speaking that an individual might adopt and would affect all elements of speech. This term however, is restricted really to vocabulary. Linguistically speaking, if a person is vulgar, it means that they tend to use obscenities. It might also refer to their selection of crass or crude conversation topics.

Comments from downvoters would be very much appreciated...
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KarlJun 14 '11 at 18:32

I don't know why people would've downvoted this answer but for me there are two surprises both of which have already come up in other people's answers: 1) That colloquial is related to region rather than related to colloquy and 2) That vulgar would only mean obscene and not rough/coarse/unrefined, though your addittion of crass/crude perhaps bridges that gap.
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hippietrailJun 14 '11 at 23:54

You're welcome ;-) As you are also welcome to correct my own typos as well whenever you find some. I know there are still a lot of them lurking in the depth of EL&U archives.
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Alain Pannetier ΦJun 15 '11 at 19:48

Informal has to do with speech or writing that is not strictly formal, or strictly standard. Colloquialism refers to informal speech or writing. Slang is a form of colloquialism, but slang isn't necessarily vulgari.e. 'dog and bone' for 'telephone', or 'bouncer' for soomething really good.

Vulgar is anything that is offensive, in formal, or informal speech. It could include slang that refers to indecent subjects i.e. 'frigging', or formal speech that refers to indecent subjects i.e.'incest'

I think of "vulgar" as the opposite of "refined" so while not the opposite of "formal" it's hard to think of a way to be both formal and vulgar. Perhaps a comical rewording of "You can stick this job up your arse!" into "Dear Sir, please insert this job anally"? (-:
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hippietrailJun 14 '11 at 0:31

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"Incest" may describe a vulgar concept, but I don't agree that it is itself a vulgar word.
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Kit Z. Fox♦Jun 14 '11 at 1:03

I'll try to think of another one. Thanks for pointing that out.
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ThursagenJun 14 '11 at 1:03

After attending an Advanced Placement College Board English Literature seminar in 2010, the presenter (sorry, can't recall her name but she was a professor at a Pennsylvania State College, maybe IUP) suggested these distinctions:

informal — what the majority of a culture uses for writing/speaking;

colloquial — words from a specific region (y'all for American South, *yinz for Pittsburgh);

slang — words from a time period (groovy for 60's, rad for 80's).

Never did get any second source to verify, but these have been good discussion points in my classroom for teaching about this topic. What do you all think?

There is some truth here, but much confusion. Yes, informal is typically used for expressions which the majority of a culture knows and uses; but specifically it means expressions which are not used in formal contexts. A colloquialism is not regional but conversational, typically with an informal context ("colloquy" means "conversation"). Slang does not mean from a time period; it means a very informal expression often known only to a specific group of people. Slang terms often have short lifetimes and so can sometimes be associated in our minds with time periods.
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MετάEdOct 15 '12 at 3:36